Puffers puff when they feel threatened. Is there someone in the tank he's not taking a liking to or perhaps another fish not taking a liking to him? Is he new? Sometimes new puffers will puff up once in awhile for the first couple of weeks until they feel comfortable in their new surroundings. Keep in mind, these animals come straight from the ocean and puffers are notorious for showing a disliking to their captivity for awhile. Most snap out of it and take to aquarium living. Others sulk in corners with tails tucked. Those are the ones that aren't happy at all. Just make sure water parameters are good and he is free of external parasites. A simple three to five minute FW
dip will help clear up any external critters. Make sure he is eating. BTW
...don't be surprised if you lose any inverts to him...including snails.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.